Add a new target independent COPY instruction and code to lower it.

The COPY instruction is intended to replace the target specific copy
instructions for virtual registers as well as the EXTRACT_SUBREG and
INSERT_SUBREG instructions in MachineFunctions. It won't we used in a selection
DAG.

COPY is lowered to native register copies by LowerSubregs.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@107529 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/CodeGen/MachineInstr.h

@@ -227,7 +227,10 @@ public:
   bool isRegSequence() const {
     return getOpcode() == TargetOpcode::REG_SEQUENCE;
   }
-  
+  bool isCopy() const {
+    return getOpcode() == TargetOpcode::COPY;
+  }
+
   /// readsRegister - Return true if the MachineInstr reads the specified
   /// register. If TargetRegisterInfo is passed, then it also checks if there
   /// is a read of a super-register.